The present invention is directed to apparatus and a method for automatically measuring the blood pressure of an individual and specifically to apparatus and a method for deflating a pressurized cuff to achieve a substantially constant rate of pressure reduction.
A conventional automatic blood pressure gauge includes a resilient inflatable cuff and an electric pump. The pump is controlled by a microprocessor to inflate the cuff with a fluid, such as air, to a preset pressure. In addition, this automatic gauge includes a pressure transducer that measures the instantaneous air pressure levels in the cuff. The pressure signal produced by the transducer is used to determine both the instantaneous air pressure of the cuff and the blood pressure pulse of the individual. This pressure signal is generally digitized and processed by the microprocessor to produce values representing the systolic and diastolic blood pressure measurements of the individual.
In operation, the cuff is affixed to the upper arm area of the patient and is then inflated to a pressure greater than the suspected systolic pressure, for example, 150 to 200 millimeters of mercury (mmHg). This pressure level collapses the main artery in the arm, effectively stopping any blood flow to the lower arm. Next, the cuff is deflated slowly and the transducer pressure signal is monitored to detect variations in cuff pressure caused by the patient's pulse, which is coupled into the cuff.
In general, the pulse component of the pressure signal has a relatively low amplitude, on the order of one percent of the total signal. A low level blood pressure pulse signal is first detected when the cuff pressure is released to a level which allows some blood flow into the collapsed artery. As the cuff deflation continues, the pulse signal rises in amplitude as more of the collapsed artery is allowed to expand in response to the pumping action of the heart. At some point, however, the pulse signal reaches a maximum amplitude level and then begins to decrease. This reduction in amplitude occurs as the artery becomes more fully open, the pumped blood flows without significantly expanding the artery, and the degree of mechanical coupling between the cuff and the arm of the patient is reduced.
In many automatic blood pressure measuring systems, the systolic and diastolic pressures are determined based on the pressure at which the pulse signal exhibits maximum amplitude. Such a system is described in U.S. Pat. No. 4,735,213 entitled DEVICE AND METHOD FOR DETERMINING SYSTOLIC BLOOD PRESSURE, which is hereby incorporated by reference for its teaching on automatic blood pressure gauges. In this system, the diastolic blood pressure is determined as the pressure, after the maximum pulse amplitude has been measured, at which the amplitude of the pulse signal is 70% of its maximum value.
Another exemplary system is described in U.S. Pat. No. 4,949,710 entitled METHOD OF ARTIFACT REJECTION FOR NONINVASIVE BLOOD-PRESSURE MEASUREMENT BY PREDICTION AND ADJUSTMENT OF BLOOD-PRESSURE DATA, which is hereby incorporated by reference for its teaching on automatic blood pressure gauges. In this system, the systolic and diastolic blood pressure levels are determined as the respective pressures corresponding to the amplitude of the pulse signal being 60% of the maximum value, prior to reaching the maximum value; and 80% of the maximum value, after reaching the maximum value.
FIG. 1a is a plot of the pressure signal versus time for a conventional automatic blood pressure gauge. This exemplary signal is generated by the cuff being quickly inflated to a preset initial pressure, greater than the systolic pressure, linearly deflated to a pressure below the diastolic pressure and then quickly deflated the rest of the way. The pulse signal is shown as a waveform superimposed on the linear deflation portion of the pressure curve. For clarity, the relative amplitude of this signal is exaggerated in the FIGURE.
FIG. 1b is a plot of the pulse signal shown in FIG. 1a, separated from the linearly decreasing pressure signal. FIG. 1c is a plot of the peak-to-peak amplitude of the signal shown in FIG. 1b. As illustrated by FIG. 1c, the amplitude of the pulse signal increases gradually until a time S, at which the linearly decreasing cuff pressure is the same as the systolic pressure of the patient. The amplitude of the pulse signal then increases at a greater rate from time S to time M at which the maximum amplitude is reached. The blood pressure level corresponding to this maximum pulse amplitude is commonly referred to as the mean arterial pressure (MAP). From this maximum amplitude, the pulse signal decreases rapidly to a time D, at which the cuff pressure is the diastolic pressure. The signal amplitude decreases from the point D until the cuff is entirely deflated.
In the second patent referenced above, a microcomputer is used to collect pressure data for a stepped reduction of cuff pressure. At each step, the cuff pressure is held constant and the signal provided by the pressure transducer is monitored for variations representing the blood pressure pulse signal. This data is then filtered to obtain a data value corresponding to a peak on the curve shown in FIG. 1b. The peak points collected over many pressure steps are fit to a curve, such as the one shown in FIG. 1c, which represents the peak-to-peak amplitudes of the waveform shown in FIG. 1b. The maximum pulse amplitude is determined from this curve.
In the referenced patent, the systolic blood pressure value is determined by locating the point S on the curve shown in FIG. 1c, prior to the occurrence of the maximum value, at which the pulse amplitude is 60% of the maximum value. The cuff pressure value which corresponds to this point is defined as the systolic pressure. Similarly, the diastolic pressure value is determined as the pressure value which corresponds to the time D in FIG. 1c. As described above, this time is after the occurrence of the maximum value of the pulse signal, when the amplitude of pulse signal is 80% of the maximum value.
In order to accurately determine the systolic and diastolic pressures of the patient, it is important to ensure that the sampling density of the points which define the curve 1b is substantially constant over the entire time that the blood pressure pulse signal is sampled. A relatively high sampling density is preferred to provide for accurate interpolation between samples and to decrease the effect of spurious "noise" pulses on the measured values. It is desirable to complete this task in a relatively short time period, so as to provide quick results and to minimize patient anxiety and discomfort. Yet, this task is complicated by differences in blood pressure from person to person and in one person within a single day. For example, the systolic blood pressure of an individual may range between 90 mmHg and 180 mmHg in a single day between periods of sleep and periods of exercise. At the same time, the diastolic pressure may range between 50 mmHg and 110 mmHg. Furthermore, the blood pressure range of children and neonates is to be considered. Consequently, to ensure that accurate measurements are made for everyone who may be tested, it is desirable for the blood pressure gauge to release cuff pressure over a wide range of cuff pressure values in a relatively short time, either in controlled steps, such as in the second referenced patent, or at a substantially constant rate, such as in the first referenced patent.
The deflation of a fixed volume container through a fixed orifice area generates a pressure deflation curve which approximates a decaying exponential. One method to obtain a linear deflation rate is to use a valve having a controllable orifice area, for example, a needle valve which can be mechanically actuated to change its orifice area. Valves of this type, however, can be difficult to control.
The size of the valve orifice may be controlled using a closed loop control system, which changes the orifice area of the valve in a manner which holds the first derivative of the measured cuff pressure value substantially constant. To minimize errors and to ensure a short settling time from pressure transients caused, for example, by patient motion, it is desirable to use a control loop having a relatively short time constant.
This type of system, however, may affect the measurement of the blood pressure pulses. It may interpret the pulse signal as a transient pressure change and attempt to compensate for it in order to maintain a constant deflation rate. This action may undesirably reduce the amplitude of some of the pulses, thus changing the shape of the pulse amplitude curve shown in FIG. 1c.
One method for overcoming this problem is disclosed in the above referenced U.S. Pat. No. 4,949,710. In this patent, a microprocessor conditions a valve to reduce cuff pressure in predefined steps. At each step, multiple pulse amplitude measurements are taken. These measurements are processed to remove artifacts and the resulting values are used to generate an interpolated curve of pulse amplitude versus cuff pressure.
The deflation process is controlled by an algorithm which predicts a range of values that indicate valid pulse amplitudes. Based on a calculated pulse amplitude curve and the measured pulse amplitude at each step, this algorithm predicts the pulse amplitude for the next step. Any detected error between the actual and predicted pulse amplitude values for the next step indicates an artifact and the erroneous pulse is ignored.
Although the referenced patent includes a predictive algorithm, it is not used to control the deflation process directly, rather it is used for artifact detection.